Java 8 stream - sum of objects

Question

Let's say I have a list of objects implementing below interface:

public interface Summable<T> {
    T add(T o1);
}

Let's say I have also some class which is able to sum these objects:

public class Calculator<T extends Summable<T>> {
    public T sum(final List<T> objects) {
        if (null == objects) {
            throw new IllegalArgumentException("Ups, list of objects cannot be null!");
        }
        T resultObject = null;
        for (T object : objects) {
            resultObject = object.add(resultObject);
        }
        return resultObject;
   }
}

How can I achieve the same using Java 8 streams?

I'm playing around a custom Collector, but couldn't figure out some neat solution.

Answer 1

list.stream().reduce(Summable::add);

interface Summable {
    Summable add(Summable o1);
}

Answer 2

What you have is a reduction:

return objects.stream().reduce(T::add).orElse(null);